The significance of the crayfish Paranephrops zealandicus as shredders in a New Zealand headwater stream

Despite their prevalence in headwater streams, crayfish are largely ignored in most leaf decomposition studies. We conducted a held survey in 30 pools along a headwater stream to document the biomass of various shredder inver tebrate species and subsequently compared their leaf processing and particu late organic matter (POM) production rates in a laboratory experiment. The crayfish Paranephrops zealnndicus dominated the shredder functional feeding group, comprising 99% on average of the total biomass of shredder inverteb rates. This was followed by the stonefly Austroperla cyrene, which made up only 0.62%. A laboratory experiment, using stoneflies and two size classes of crayfish, showed that processing rates of large crayfish (normalised by body weight) were lower than those of small crayfish and stoneflies. Howeve r, large crayfish had the greatest impact on leaf decomposition and POM pro duction. It is their large body size and dominance of invertebrate biomass that determines the significance of crayfish as shredders. Given their wide spread distribution in New Zealand and elsewhere, crayfish may play key ro les as shredders in many headwater streams. especially where shredder insec t diversity/abundance is low.